Resistance to anthelmintic drug therapy is occassionally observed in the wild but rarely developed in the laboratory. Egerton et al., J. Parasit. 24 614-617 (1988) discloses a laboratory derived strain of Haemonchus contortus resistant to ivermectin. Giordano et al, Veterinary Parasitology 30 139-148 (1988), discloses a strain of T. colubriformis which is only partially resistant to ivermectin. No disclosures are known of any laboratory derived strains of helminths resistant to both ivermectin and thiabendazole. These characteristics of the new strain of T. colubriformis make it particularly useful as an agent for the selection of antihelmintic compounds with novel modes of action.